1. Field of the Invention
An early method for working with a high frequency carrier signal is disclosed in the German patent application DE 196 01 866.
Various methods and devices are known to transmit signals between a medical device, in particular an implanted one, and an external transmitter or receiver. For example, modern cardiac pacemakers can record an intracardial electrocardiogram (IECG) using the pacemaker electrodes and can transmit it, using a telemetry unit, to an extracorporeal control device.
2. Description of Related Art
In modern signal transfer methods that are known for implanted cardiac pacemakers, e.g. from the book by John G. Webster (Editor): xe2x80x9cDesign of Cardiac Pacemakersxe2x80x9d, section 12 xe2x80x9cExternal Programmingxe2x80x9d, IEEE Press Book Series, New York 1995, the digital signal that is to be transferred wirelessly is modulated onto the high frequency carrier signal in bit sequences by a modulator in the transmitter. It is then transmitted across a distance to the receiver, which contains a corresponding demodulator for recovery of the data signal. The carrier signal is in a comparatively low frequency range, since it has to penetrate the body and must not interfere with neighbouring medical devices.
All such methods have the disadvantage that the quality of the data signal that is recovered on the receiver side strongly deteriorates with the distance between transmitter and receiver, and with interference in the transmission path.
The transmitting power must not fall below a definite value, so that a desired range with a prescribed certain noise immunity can be achieved in an information transfer over a noisy transmission path.
This required high transmitting power, on one hand, has the disadvantage that the energy consumption during the transmitting operation is correspondingly high, which is of disadvantage for battery operated devices, such as the previously mentioned cardiac pacemakers, due to rapid battery exhaustion. On the other hand, one is concerned that the electromagnetic radiation emitted from the transmitter can lead to harm to the human body, which must in particular be considered for implanted medical devices due to the extremely low distance from the patient.
The objective of this invention is to create a method of the previously mentioned type and an arrangement for the implementation of that method, which allows a lowering of the transmitting power and an increase in range for medical implantsxe2x80x94while at least maintaining the transmission quality.
The invention incorporates the technical principle, to subject the pulses, modulated with the information, using a known method of telecommunications, to an angle modulation in the transmitter. (Angle modulation is to be read as a generic term for phase and frequency modulation) These angle modulated pulses are time compressed in the receiver by introducing a time delay using suitable means, so that the duration of the pulses is shortened and they experience an amplitude enhancement. This pulse compression can be carried out using a dispersion filter. The information can be recovered from the pulses processed in this manner by a corresponding demodulation, whereby the demodulation can be carried out with an improved signal/noise ratio, due to the increase in amplitude. The actual information can be imprinted onto the pulse by a pulse modulation method, or by carrying out the pulse compression in a discernibly different manner for pulses sequential in time, so that the information is contained in this variation of the angle modulation.
Thus a signal is available after the demodulation, that otherwise could only be obtained by using higher transmitting power, if not using any other costly methods to improve reception, such as diversity reception or signal encoding, which occupies a larger frequency range or a longer transmission time due to redundant components, so that the available data channel would show a lower data throughput or could only be used by a lower number of users.
In this invention, the angle modulation of the pulses in the transmitter is carried out according to a modulation that, during the pulse duration, determines a change in frequency, in case of a frequency modulation, or a shift in phase, in case of a phase modulation. Phase and frequency modulation are both treated under the common generic term of angle modulation.
While the modulation of the pulses can be achieved using different pulse modulation methods, in the variable angle modulation a special angle modulation time characteristic is used, corresponding to a xe2x80x9cmodulation characteristic curvexe2x80x9d.
Hereby, the modulation characteristic curvexe2x80x94here referred to as modulation characteristicxe2x80x94determines the time behaviour of the frequency during the pulse duration. Preferably, the frequency of the transferred signal decreases linearly during the pulse duration, from a value above the carrier frequency to a value below the carrier frequency. The filter on the receiver side is matched to the employed modulation characteristic by a corresponding differential, frequency dependent delay time response, in such a manner so that the generated signal components of different phase position superpose to form a nearly coincident signal.
The imprinting of the information to be transmitted can occur either by varying or selecting the modulation characteristic, or by any other conventional modulation method that has no effect on the signal delay time, or only to a secondary degree. A preferred option is the modification of the amplitude of the transmitted signal dependent on the input signalxe2x80x94i.e. amplitude modulation, or all types of encoding in which the transmitted information is determined by the type, number, position, or sequence of the transferred pulses.
The invention offers in an advantageous manner the possibility to transmit signals to devices, in particular implanted ones, using higher frequencies than customary until now, without affecting the tissue on one hand, and without electromagnetic interference (EMI) to other devices used in the clinical environment on the other hand. Until now this was the main problem in the use of devices emitting electromagnetic waves in clinical surroundings. Until now these conditions ruled out, for example, the use of portable telephones etc. Additionally, this invention""s method offers the advantage that a signal transfer can be made across larger distances (for example within a patient""s room), so that programming devices etc. do not have to be attached directly to the patient""s body. When appropriate codes are selected, it is also possible to communicate in parallel with several devices without mutual interference. Since the used signals can be transmitted with low amplitude, they do not rise above the surrounding noise level, or only negligibly. Thus the mutual interaction between them is low.
In a preferred embodiment of the invention the imprinting of the information of the input signal occurs by selecting or modifying a modulation characteristic dependent on the input signal. If the input signal has a high-level, then, for example, a modulation characteristic linearly falling with the signal is used, which leads to a frequency modulated pulse in which the frequency decreases during the pulse duration. For a low-level of the input signal a linearly rising modulation characteristic is used, which correspondingly leads to a pulse with frequency that increases during the pulse duration. The filter means on the receiver side are appropriately matched.
The invention is not limited to linear modulation characteristics, but can be implemented with modulation characteristics of any shape, whereby it is only necessary to assign distinct modulations to different levels of the input signals, so that a subsequent signal discrimination is possible in the receiver.
It is also possible to use more than two modulation characteristics for the input signal, so that every pulse transmits a larger information content. If, for example, four different modulation characteristics are available, then correspondingly four different pulses can be transmitted, which corresponds to a data content of 2 bits for each of the transmitted pulses. By increasing the number of distinct modulation characteristics the data transfer rate can be increased advantageously, whereby it must be noted that it becomes more difficult to distinguish between the frequency modulated pulses when a very large number of modulation characteristics are used, which increases the transfer""s susceptibility to errors.
In the previously described embodiment of the invention the modulation of pulses occurs actively for both a high-level as well as for a low-level of the digital input signal. This means that during a low-level and a high-level of the input signal, frequency modulated pulses are generated, that are distinguished by the frequency change during the pulse duration. Thus hereby, the imprinting of the information contained in the input signal onto the transferred signal is achieved through selection or variation of the modulation characteristic depending on the input signal.
In another variation of the invention, the angle modulation of the pulses in the transmitter occurs independently of the input signal to be transmitted, according to a single default modulation characteristic, which determines the variation of frequency or phase during the duration of a pulse. The imprinting of the information contained in the input signal onto the transmitting signal can be effected in various ways, according to well known digital modulation methods. It is favourable to carry out a pulse position modulation (PPM), in which the position of the individual frequency modulated pulses is modified depending on the input signal.
In a preferred embodiment of the invention, the imprinting of the information contained in the input signal onto the transmitting signal is effected by pulse code modulation (PCM), in which the sequence of the pulses to be transferred is modified depending on the input signal. For a digital input signal the transfer of the input signal occurs actively only for one level, whereas no pulse is generated for the other level, so that the different pulses are only distinguished by their amplitude. For a high level of the input signal preferably a linearly rising frequency modulated pulse is generated, while for a low level a pause with the length of the pulse is inserted. This variation of the invention allows implementing a modulation of the pulses of the digital input signal with only one modulation characteristic.
In this present design for imprinting the information contained in the input signal onto the transmitting signal, the invention is not limited to the previously mentioned pulse position modulation or pulse code modulation, but can in principle be implemented with all known digital modulation methods.
The transmitter transfers the signal, frequency modulated by one of the previously described methods, across the transmission path to a receiver, where it is demodulated to recover the data signal.
Here, and in the following, the term transmission path should be taken generally, as comprising all wireless transmission paths in which the data transfer from the transmitter to the receiver occurs by means of electromagnetic waves.
To be able to distinguish the frequency modulated pulses, generated by the transmitter, from noise signals in the receiver, these pulses are compressed in the receiver, which leads to a corresponding increase in amplitude by increasing the signal/noise ratio.
A further advantage of this invention""s method is a significantly lower interference potential compared to other transmitters and receivers, because a predetermined signal to noise ratio can be achieved with a lower transmitting power after the pulse compression in the receiver. In addition, the lower demands on the transmitting power lead to a lowered environmental impact by electromagnetic radiation.
To compress the pulses picked up on the receiver side, which are frequency modulated according to the modulation characteristic used by the transmitter, the received signal is filtered by a dispersion filter with a predetermined, frequency dependent, differential delay time response.
In the invention""s variation that uses only a single modulation characteristic for generating a frequency modulated pulse on the transmitter side, described above, only a single dispersion filter is required on the receiver side, whereby the frequency dependent delay time response of this dispersion filter is matched to the modulation characteristic of the angle modulation carried out on the transmitter side in such a way, that the spectral signal components of the frequency modulated pulse generated on the transmitter side arrive essentially coincident at the output of the dispersion filter, which leads to a pulse compression and a corresponding increase in amplitude. If the angle modulation on the transmitter side is effected according to a linearly falling modulation characteristic, then the frequency of the pulse decreases during the pulse duration, which results in an arrival at the receiver of the high frequency signal components side before the low frequency signal components. The delay time response of the dispersion filter on the receiver side must compensate for this xe2x80x9cleadxe2x80x9d of the high frequency signal components, so that the spectral signal components of the frequency modulated pulse superpose to form a pulse with increased amplitude at the output of the dispersion filter.
The recovery of the information contained in the input signal is carried out by a detector connected after the dispersion filter, which is matched to the modulation method, that is used on the transmitter side for imprinting the information contained in the input signal.
If, depending on the amplitude of the input signal, one of several modulation characteristics is selected on the transmitter side, preferably a linearly falling modulation characteristic for a high-level, and a linearly rising modulation characteristic for a low-level of the input signal, then fundamentally two options exist for the interpretation in the receiver.
One option is to provide only one dispersion filter on the receiver side, the delay time response of which is matched to the modulation characteristic used on the transmitter side, in a manner so that the spectral signal components of the pulse, frequency modulated according to this modulation characteristic, arrive essentially coincident at the output of the dispersion filter, which leads to a pulse compression and increase in amplitude. If the frequency modulation on the transmitter side occurs according to one of the other modulation characteristics that are not optimally matched to the delay time response of the dispersion filter on the receiver side, then the spectral signal components of the frequency modulated pulse arrive at the output of the dispersion filter distributed over time, and thus, due to the lower pulse compression or expansion, also with a smaller amplitude. In this embodiment, the amplitude of the pulse that arrives at the output of the dispersion filter depends on the modulation characteristic used at the transmitter side, and thus on the amplitude of the input signal employed in the selection of the modulation characteristic. A detector that can be executed, for example, as an amplitude demodulator, is connected after the dispersion filter, to recover the digital input signal from the output signal of the dispersion filter.
In the other option the frequency modulated pulse is fed to several dispersion filters on the receiver side. The differential delay time response of the dispersion filters arranged on the receiver side and the modulation characteristics used on the transmitter side are hereby matched in pairs in such a way, that the spectral signal components of the frequency modulated pulse arrive essentially coincident at the output of exactly one of the dispersion filters, thus leading to an increase in amplitude, while the output signals of the other dispersion filters are not increased due to the differing characteristics. Thus the input signal can be discriminated according to which dispersion filter shows an increase in amplitude.
Advantageously, surface acoustic wave filters (English: SAW- Filter: Surface Acoustic Waves) are used as dispersion filters. Hereby a dispersion filter shows a frequency dependent, differential delay time response that is matched to the angle modulation carried out on the transmitter side, in such a way that the different spectral components of the transmitted signal arrive nearly coincident at the output of the dispersion filter in the receiver, due to their different transit times through the dispersion filter, so that the output amplitude is strongly increased by optimum superposition of the spectral components.
The generation of the frequency modulated signal in the transmitter can be achieved in various ways, some of which are briefly described in the following.
In a preferred embodiment of the invention, at first an approximate (quasi-) Dirac pulse is generated and fed to a low-pass filter, the filter characteristic of which shows a peak just before the critical frequency, and thus transforms the delta impulse to a Sinc-pulse, the shape of which is described by the Sinc-function Sinc(x)=sin(x)/x. The Sinc-shaped output signal of the low-pass filter subsequently is fed to an amplitude modulator that imprints a Sinc-shaped envelope onto the carrier oscillation. When the signal generated in this manner is fed to a dispersive filter, a frequency modulated pulse appears at its output. Thus in this variation of the invention, on the transmitter side the dispersion filter at first expands the relatively sharp Sinc-impulse into a frequency modulated pulse, that is wider, compared to the Sinc-pulse, and possesses a correspondingly lower amplitude. On the receiver side, a dispersion filter effects a compression of the pulse with a corresponding increase in amplitude. Since one dispersion filter each is used for the expansion of the pulses on the transmitter side, and the compression on the receiver side, this variation is advantageously suited for a transceiver operation with alternating transmitting and receiving operation. For this, transmitter and receiver can each contain corresponding identical component modules, with one dispersion filter each, that are used for the generation of the frequency modulated pulse in transmitting operation, and for the compression of the received frequency modulated pulses in receiving mode.
In another variation of the invention, the generation of the frequency modulated pulses is effected using a PLL (PLL: Phase Locked Loop) and a voltage controlled oscillator (VCO: Voltage Controlled Oscillator). The individual pulses of the input signal that is present in digital form are hereby at first converted to saw-tooth shaped pulses in an integrator, whereby the direction of the rise of the individual pulses depends on the amplitude of the input signal. The signal generated in this manner is then used for controlling the VCO, so that the frequency of the output pulse linearly increases or decreases during the pulse duration, depending on the level of the input signal.
In a further variation of the invention, the generation of the frequency modulated pulse in the transmitter is effected by a digital signal-processing unit, which advantageously allows the implementation of any desired modulation characteristics.
In a message transfer system according to this invention, it is necessary to match the frequency dependent delay time response of the dispersion. filter used on the receiver side to the modulation characteristic of the frequency modulation carried out on the transmitter side, so that a pulse compression in the receiver can be achieved.
In a variation of the invention, matched transmitter-receiver pairs are produced for this purpose, so that no further tuning work is necessary when the system is brought into service. The previously mentioned dispersion filters preferably are executed as surface acoustic wave filters (SAW-Filter: Surface Acoustic Waves), since such filters can be produced with high accuracy and stability. In addition, such surface acoustic wave filters offer the advantage that amplitude response and phase response can be dimensioned independently of each other, which offers the possibility of implementing the narrow-band band-pass filter that is required in each receiver and the dispersion filter in one component. Such filters are known for other application areas, for example from the European patent application EP 0 0223 554 A2.
In another variation of the invention, the receiver is matched to the transmitter by varying the delay time response of the dispersion filter used on the receiver side.
Thus in one advantageous variation of the invention, the transmitter, during a matching process, emits a reference signal that preferably corresponds to a sequence of high-levels of the input signal, whereby the modulation characteristic of the frequency modulation carried out on the transmitter side, or the frequency dependent delay time response of the dispersion filter on the receiver side, are varied, until an optimum pulse compression and increase in amplitude is achieved on the receiver side. This variation is especially advantageous when using a digital signal processor for filtering and processing in the receiver, since such a signal processor in a simple manner allows a modification of the frequency dependent delay time response and a corresponding optimization, whereby the optimization process can be executed automatically using computer control.
In a further advantageous embodiment of this variation, the data transfer occurs block by block, whereby the matching process described above is carried out again for each block, to be able to dynamically compensate for fluctuations of the dispersion characteristics of the transmission path.
Other advantageous, further developments of the invention are illustrated in more detail in the following figures together with the description of the invention""s preferred embodiment. The figures show: